This invention relates to aqueous blends comprising at least two electrically conducting conjugated polymers and films formed from such aqueous blends. The blends and films are well suited for use in a variety of applications including but not limited to fabricating the hole injection/transporting layer of photovoltaic devices and electroluminescent devices.
Conjugated polymers were initially observed to exhibit electrical conductivity by Shirakawa et al., F. Chem. Soc., Chem. Commun. P.578 (1977); Chiang et al., Phys. Rev. Lett., 39, 1098 (1977); and Chiang et al., J. Am. Chem. Soc., 100,1013 (1978). The researchers observed that poly(acetylene) yielded an electrically conducting polymer when doped with an oxidizing or a reducing agent. The conjugated polymer as oxidized is electron deficient (positively charged) and thus neutralized with an anion (the reverse situation with reducing agent doping). Once recognized that the conjugation existing in poly(acetylene) had similarities to many other conjugated polymers, a major research effort was initiated.
Other conjugated polymers that exhibit electrically conducting properties upon being doped include poly(thiophenes), poly(anilines), poly(pyrroles), poly(phenylenes), poly(phenylene vinylenes) (PPV), poly(thienyl vinylenes) and poly(phenylene sulfides). The conductivities of undoped conjugated polymers are generally in the range of 10−7 to 10−10 S/cm (i.e., in the semiconductive region and close to the insulating region).
Initial research involved iodine doped poly(acetylene) yielding electrical conductivities of up to 103 S/cm (see Shirakawa et al., Chiang et al., (1977); and Chiang et al., (1978)). Improved conductivities of 104 S/cm and 105 S/cm were reported in later studies by Naarmann and Theophilou in Synth. Met., 22, 1 (1987) and by Tsukonoto in Adv. Phys., 41, 509 (1992) respectively. A ClO4− doped poly(acetylene) with a conductivity of 40,000 S/cm was reported by Park et al., Synthetic Metals, 96, 81 (1998).
Limitations of processibility and thermal/oxidative stability of poly(acetylene) led to studies on other conjugated polymers. Menon et al., Handbook of Conducting Polymers, 2nd edition, edited by T. A. Skotheim, R. L. Elsenbaumer and J. R. Reynolds, Marcel Dekker, New York, 1998 summarized the state-of-the-art and noted references showing PF6 doped oriented poly(pyrrole) with an electrical conductivity of 103 S/cm, doped poly(aniline) with electrical conductivity of 300-400 S/cm and regioregular poly(alkylthiophenes) having electrical conductivities approaching 103 S/cm.
Poly(acetylene) is unique among most conducting polymers because it can be either p-doped (acid; electron acceptors) or n-doped (base) to obtain effective conductivities. The conductivity of potassium doped poly(acetylene) has been reported as 4000 S/cm at atmospheric pressure and 8000 S/cm at 10 kbar pressure by K. Vakiparta et al., Phys. Rev. B, 47, 9977 (1993).
Ahlskog et al., J. of Phys.: Condensed Matter, 9, 4145 (1997) reported conductivity values of various doped conjugated polymers as shown below:
PolymerDopantConductivity (S/cm)Poly(acetylene)I2>5 × 104Poly(acetylene)FeCl3>2 × 104Poly(phenylene vinylene)AsF5  300-2400Poly(phenylene vinylene)H2SO4   4000-10,000Poly(pyrrole)not specified  300-400Poly(aniline)not specified  250-350
A wide variety of dopants have been used to dope various conjugated polymers. Primarily p-type dopants (acid; electron acceptors) have been studied. Acid dopants include mineral acids such as HCl, HNO3, H2SO4 and organic sulfonic acids including camphor sulfonic acid, lauryl sulfonic acid, dodecyl benzene sulfonic acid, toluene sulfonic acid, 2-acrylamido-2-methyl-1-propane sulfonic acid, methane sulfonic acid, various organic aromatic sulfonic acid dyes and, to a lesser degree, carboxylic acids. Other p-type dopants include PF6, AsF5, FeCl3, SO3, BF3, MoCl5 and ZnNO3. Polymeric dopants include poly(sulfonic acids) such as poly(styrene sulfonic acid), amic acid presursors of polyimides, poly(carboxylic acids) such as poly(acrylic acid). Weaker acids (such as carboxylic acids) generally yield limited electrically conductivity enhancement and are rarely studied. In some cases, the polymers are self-doped such as sulfonated poly(aniline) (SPANI).
Additives have been used to improve the conductivity of doped conjugated polymers. A secondary doping was observed for poly(aniline) with the addition of m-cresol yielding an increase in conductivity and crystallinity of poly(aniline) (see Y. Cao et al. Synth. Met., 48, 91 (1992)). MacDiarmid and Epstein noted that m-cresol is a secondary dopant for camphor sulfonic acid doped poly(aniline) (Synth. Met., 65,103 (1994)). Studies showed that camphor sulfonic acid was only a fair dopant for poly(aniline), and that higher conductivities can be obtained with the addition of m-cresol. Blends of a water soluble precursor to PPV with other water soluble polymers such as poly(ethylene oxide) (PEO), poly(vinyl methyl ether) (PVME), methyl cellulose (MC), hydroxypropyl cellulose (HPC) and poly(vinyl pyrrolidone) (PVP) were compared with PPV (after conversion of the water soluble precursor to PPV and doping with AsF5). 50/50 blends showed increased conductivity over the control PPV for PEO, PVME and HPC blends with PPV. PEO gave the best results with 166 S/cm over the control 26 S/cm (J. B. Schlenoff et al., J. Polym. Sci., Part B: Polym. Phys., 26, 2247(1988)).
Compositions involving mixed dopants have been described by S. H. Kim et al., in J. Appl. Polym. Sci., 83, 2245 (2002). Poly(aniline) coated NOMEX conducting fibers (DuPont, Wilmington, Del.) were evaluated with HCl, camphor sulfonic acid, toluene sulfonic acid, benzene sulfonic acid, dodecylbenzene sulfonic acid and various blends of HCl with the organic sulfonic acids. The researchers observed that conjugated polymers treated with mixed dopants generally gave higher conductivities than conjugated polymers doped with a single dopant.
Blends of doped conjugated polymers with insulating polymers have been disclosed in the literature. Heeger and coworkers have described conductive polymer blends with excellent efficiency (see Y. Cao et al., Appl. Phys. Lett., 60(22), 2711 (1992) and C. Y. Yang et al., Synth. Met., 53, 293 (1993)). Anionic surfactants were employed as dopants for poly(aniline). Mixtures of the surfactant doped poly(aniline) with insulating polymers cast from mutual solvents exhibited threshold conductivities at compositions as low as 1 wt % poly(aniline). The network structure exhibited an inter-connective morphology for the doped poly(aniline) as contrasted to a particulate dispersed structure that would generally be expected. Other polymers including poly(carbonate), poly(sulfone) and poly(ethylene) showed this behavior at low levels of the surfactant doped poly(aniline) addition. Latex blends comprising electrically conductive polymers have been shown to exhibit electrical conductivity at low levels of addition of a doped conjugated polymer.
Segal et al. disclosed that poly(aniline) (PANI) doped with dodecyl benzene sulfonic acid (DBSA) gave a percolation threshold for conductivity as low as 0.5 wt % (J. Appl. Polym. Sci., 79, 760 (2001)).
Conductivity measurements on blends of doped conjugated polymers with other conducting polymers have not been studied before. Several reports discuss the electrical conductivity of “polypyrrole/polyaniline blends” (see S. Sakkopoulos et al., J. Materials Science, 37, 2865 (2002) and G. S. Akundy et al., J. Appl. Polym. Sci., 833, 1970 (2002)). However, it is evident from the preparation procedure of these “blends,” that pyrrole and aniline monomers were mixed prior to polymerization, thus the resulting “blends” were most likely copolymers. The conductivity of non-aged samples versus composition exhibited scattered behavior for FeCl3 polymerized samples (FeCl3 was used as a dopant).
A review of conductive polymer blends by DePaoli et al. did not mention any blends of doped conductive polymers (Macromol. Symp. 189, 83 (2002)).
S. K. M. Jonsson et al., Synth. Met., 10361, 1 (2003) disclose effects of solvent and thermal treatment on the conductivity of poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT-PSSA). The researchers found that specific solvents and polyhydric alcohols (e.g., sorbitol) can yield an increase in conductivity up to a factor of 600 over the control PEDOT-PSSA.
Despite the foregoing developments, there is a need in the art for blends that can be cast in aqueous media and films formed from such blends which provide improved conductivity and processibility.